The present invention relates to a process for fabricating an accelerometer of a type having no proof or inertial mass and no moving parts or parts under stress such as piezo or strain gauge accelerometers.
Accelerometers find use in widely diverse applications including automobile air bags and suspension systems, computer hard disc drivers, smart detonation systems for bombs and missiles and machine vibration monitors. Silicon micromachined acceleration sensors are beginning to replace mechanical acceleration switches. Present accelerometers are all based upon the classical Newtonian relationship of force, F, mass, m, and acceleration, a, in which F=ma. Thus, for a cantilevered beam, the force due to acceleration causes the beam to deflect. This deflection is sensed either by sensing the change in piezo resistance or by a change in capacitance. Such systems are not stable over wide temperature ranges and have a response which peaks due to insufficient mechanical damping.
One form of accelerometer made by bulk micromachining consists of membrane or diaphragm of silicon formed by chemical etching having a large mass of silicon at the centre and tethers of thin film piezo-resistors, whose resistance is sensitive to strain and deformation, suspending the mass. Acceleration causes the large silicon mass to move, deforming the diaphragm and changing the resistance of the piezo-resistors. Such bulk micromachined devices are large by integrated circuit standards and consistent with semiconductor circuit fabrication techniques.
Another system made by surface micromachining is based on a differential capacitor. Surface micromachining creates much smaller, more intricate and precisely patterned structures than those made by bulk micromachining. It involves the same process that is used to make integrated circuits, namely, depositing and etching multiple thin films and layers of silicon and silicon-oxide to form complex mechanical structures. In this case a central beam is affixed in an xe2x80x9cHxe2x80x9d configuration with the spaced apart parallel arms of the xe2x80x9cHxe2x80x9d supporting respective ends of the cross beam.
A plate affixed perpendicular to the beam forms a moving capacitor plate that is positioned between two fixed plates, thus, forming two capacitors sharing a common moving plate. When the unit is subjected to an accelerating force the beam and hence moving plate moves closer to one of the fixed plates and away from the other fixed plate. The effect is to reduce one of the capacitors and increase the other by an amount proportional to the acceleration. The device requires proper orientation with the cross beam parallel to the direction of acceleration. However, surface micromachining is used to create a much smaller device adapted to the same techniques used to make integrated circuits. The moving capacitor plate accelerometer suffers from high noise and exhibits drift at low acceleration measurements.
It is an object of the present invention to provide an improved accelerometer. It is a further object of the invention to provide an accelerometer having no proof mass and a corresponding increase in ruggedness.
According to the invention there is provided a process for fabricating an accelerometer which includes providing a substrate with a layer of electrically conductive material on the substrate, micromachining the electrically conductive material to form a primary heater and a pair of temperature sensitive elements, one located on each side of and spaced apart from said electrically conductive primary heater a distance in the range of 75 to 400 microns, and micromachining the substrate to form a cavity below the heater and the temperature sensitive elements, thereby forming the accelerometer.
In Applicant""s parent application a spacing between each temperature sensitive element and the primary heater was 20 microns. A vastly improved sensitivity is realized by increasing this spacing to be in the range of 75 to 400 microns.
In yet another aspect of the invention there is provided a process for fabricating an accelerometer which includes heating an n-type silicon substrate at a dielectric forming temperature sufficiently high to form a first dielectric upon the substrate. This step is followed by depositing a layer of electrically conductive material over the first dielectric layer. Next a second dielectric layer is formed over the layer of electrically conductive material, and the second dielectric layer is patterned over the layer of electrically conductive material to form three spaced apart bridges. The layer of electrically conductive material is etched using the second dielectric layer as a mask down to the first dielectric layer covering the substrate, such that a central bridge of the three spaced apart bridges of electrically conductive material corresponds to a primary electric heater and the other two of said bridges correspond to a pair of temperature sensing elements, one on each side of the primary electric heater and spaced from said central heater a distance of 75 to 400 microns. Next the substrate is heated so as to oxidize the side walls of the electrically conductive material in the bridges, patterning and etching the first and second dielectric layer above and below said bridges to create openings for bonding pads and to expose said substrate for formation of a space below said bridges and finally a space below said bridges is formed by patterning and etching.